1 Field of the Invention
This invention relates to an arrangement for controlling the temperature performance characteristics in a manufacturing process. More particularly, this invention relates to an enhancement in a temperature control system by use of a variable frequency pulse generation technique, this temperature control system being used in conjunction with an extruder process for extruding a plastic or other type of coating substance onto a material to be coated as for instance, a paper material.
2 Description of the Prior Art
For automated industrial processes in use today, particularly in systems where it is necessary to maintain predictable operating performances over a large enough time frame so as to greatly reduce or even eliminate system downtime and product waste, precise control over the operating parameters of the devices which actually perform the manufacturing operation, has been accomplished using complex distributive processing techniques. One such distributive processing system has been effectively utilized in a material coating operation such as for instance, a plastic film coating operation for a paper mill system wherein it is necessary to maintain the operating temperature of the extruder barrel to a precise degree so as to insure that the proper thickness of plastic film is dispersed onto the paper product at all times. It can be appreciated that a temperature control apparatus for an extruding process has widespread applicability in other areas besides the above example and that this invention is intended to cover those areas as well, an example of which can be a rubber coating operation. Furthermore, a temperature control apparatus such as is disclosed herein, can be used in other industrial process applications as well, an example of which can be found in U.S. Pat. No. 3,437,325 issued to R. E. Putman, Jr. on Apr. 8, 1969 which is directed to an arrangement for balancing the heat control in a rotary kiln.
One paper coating industrial operation using an extrusion process is disclosed in U.S. Pat. No. 3,161,560 issued on Dec. 15, 1964 to Owen D. Mosher et al. Another industrial operation involving the application of a polyethylene film to a paper product, found in U.S. Pat. No. 4,222,973 issued to John A. Hill et al. on Sept. 16, 1980, illustrates the importance of maintaining the extrusion melt temperature to within a predetermined range. Yet another application of a paper coating operation that recognizes the significance of the extrusion melt temperature in maintaining the thickness of the extrusion coating to within a predetermined dimension is found in U.S. Reissue Pat. No. RE-32270 which was reissued to Lee J. Murray, Jr. on Oct. 28, 1986.
The extruding process associated with the coating of the paper material must be controlled so that the thickness of the coating material is uniform over the entire length of the paper product and for all phases of operation of the paper processing operation whether during system start-up, everyday production, or shut-down of the system. In this particular application, a sheet of the paper product passes beneath an extruder barrel which can have associated therewith, a number of zones each of which can be controlled as to the amount of coating material dispersed therethrough. Each zone must be controlled in the similar manner so that the overall coating operation is uniform.
Certain factors establish the flow of coating material through the extruder barrel, the two primary factors of which are the speed of the screw disposed within the extruder barrel to perform the function of extruding the coating material, and the other of which is the temperature of the extruder barrel which, in dealing with a plastic coating material, is of inherent importance. To illustrate this importance, it has been found that one problem encountered in the start-up operation of the paper coating process has been that an overshoot of the process temperature of the extruder barrel has caused either an excess or a deficiency in the amount of coating material to be dispersed onto the paper product thus ruining that production run. Temperature overshoot can be characterized as a damped oscillation condition and is defined as that condition which occurs when the selected temperature setting is not immediately achieved; that is, the actual process temperature varies about the setpoint temperature for a period of time before it settles into the selected setpoint temperature. It is understood that the problem of temperature overshoot is not limited to the start-up sequence, but can occur at any time it is desired to change the extruder through-put which is the amount of coating material extruded through the extruder barrel. It is further required that the temperature control apparatus for the extruder process be effective for controlling this temperature overshoot problem for all screw speeds.
Another problem encountered in existing extruding processes associated with a paper coating manufacturing operation comes about as a result of a temperature sag that occurs during an increase in extruder through-put. It is known that the practice of increasing extruder through-put is accomplished by increasing the extruder screw speed and that at this time the temperature of the extruder barrel which has been previously selected to accommodate a specific amount of coating material, cannot maintain the setpoint temperature for the increased volume. It is therefore required that a temperature control apparatus for an extruding process compensate for this temperature sag during periods of increasing extruder through-put so that the coating material thickness can be maintained to the preselected dimension.
Inherent in the control of the operating temperature of the extruding process is the control of the heating and/or cooling devices which are disposed in proximate relation to the extruder barrel. Altlhough the present temperature control system has been described in conjunction with a distributive processing system which may be used to control an entire production facility, it is a further requirement of the present invention that the temperature control apparatus be operable in a situation where the extruder process is implemented using a standard electrical hardware configuration. For this type of configuration, the output signal of the temperature control apparatus must be compatible with the operation of the heating and cooling devices.